There are a bunch of reasons that you shouldn't have a website that is just one big flash application. Here is one simple one. The other day we were thinking of going to Tejas for lunch and wanted to check their hours. Simple enough. So I brought up their website on my iPhone. This is what I got.
Needless to say, totally and completely useless. In fact, it took me a while to even realize if I was on the right website. At least have the header, or something at all, in HTML! We went somewhere else.